The present invention relates to a ductility additive for use in tungsten alloy electroplating baths which provides tungsten alloy electroplates for use in replacing hexavalent chromium plating or other hard lubrous coatings.
Chromium plating for decorative and functional plating purposes has always been desirable. Most often chromium plating is carried out in hexavalent chromium electrolytes. Functional coatings from hexavalent chromium baths generally range in thickness from about 0.0002" to about 0.200" and provide very hard, lubrous corrosion resistant coatings. Decorative coatings from hexavalent chromium electrolytes are much thinner, typically 0.000005" to 0.000030", and are desirable because of their blue-white color, and abrasion and tarnish resistance. These coatings are almost always plated over decorative nickel or cobalt, or nickel alloys containing cobalt or iron.
The imposition of government restrictions on the discharge of toxic effluent, including hexavalent chromium present in conventional chromium plating baths, has escalated in recent years. Some state and local government restrictions are extremely stringent. This is especially the case with regard to fumes generated during the electrolysis of hexavalent chromium baths. In some locales, even minuscule amounts of airborne chromium is unacceptable. This has prompted the development of alternative electroplating baths intended to approach the color and the characteristics of chromium deposits.
One possible solution is the electrodeposition of tungsten alloys. Typically, in such baths, salts of nickel, cobalt, iron or mixtures thereof are used in combination with tungsten salts to produce tungsten alloy deposits on various conductive substrates. In this case, the nickel, cobalt and/or iron ions act to catalyze the deposition of tungsten, such that alloys containing as much as 50% tungsten can be deposited; said deposits having excellent abrasion resistance, hardness, lubricity and acceptable color when compared to chromium.
However, while such deposits have been desirable as replacements for chromium, the properties of resulting deposits and inherent manufacturing limitations in prior art processes have not allowed such deposits to replace decorative or functional chromium deposits. While alkaline complexed nickel-tungsten co-deposits have been known, the deposits produced from these electrolytes often are generally low in ductility and, therefore, are subject to stress cracking and the like. Thus, use of tungsten electroplates has been limited to thin deposits or deposits where cracks are allowed.
Commonly assigned prior U.S. Pat. No. 5,525,206 to Wieczerniak addresses brightening agents for improving surface and appearance qualities. However, there remains a need in the art to provide tungsten alloy electroplates with improved physical properties of ductility.